Operator control of medical imaging appliances may involve the use of “touch panels” (e.g., control panels) having a resistive touch screen (e.g., touch-sensitive screen). The safety requirements of European Medical Safety Standard EN60601, inter alia, are to be met.
Resistive touch screens react to pressure that locally connects two electrically conductive layers. The layers thus form a voltage divider that is used to measure the electrical resistance in order to ascertain the position of the pressure location. Such touch screens include an outer polyester layer and an inner glass or plastic plate that are separated from one another by spacers. The areas facing one another are coated with indium tin oxide, a transparent semiconductor.
In order to ascertain the position of the pressure location, DC voltage is applied to one of the conductive layers. The voltage drops uniformly from one edge of the layer to the opposite edge. At the pressure location, the voltage on both layers is the same because the conductive layers are connected at the pressure location point. The second conductive layer is the connection of the pressure location point to the outside. Two voltages may be measured between the edge of the second layer and the two opposite edges of the first layer. If the two voltages are the same, the pressure point is exactly in the center between the two edges of the first layer. The higher one voltage is in relation to the other, the further away the pressure point is from the respective edge.
A second measurement is performed with the roles of the two layers reversed, so that the distances to the other two edges may be ascertained. Then, the position in the area is established. In order to record both dimensions, the DC voltage is thus applied alternately crosswise. Errors in the actuating electronics of the touch screen may lead to erroneous position finding on the screen that may ultimately lead to dangerous states in the case of a medical imaging appliance. By way of example, errors in the actuating circuit leading to a position at the outermost edge of the touch screen may be erroneously measured instead of the correct position.